1. Technical Field
The present invention relates to orthopedic prostheses and, specifically, to knee prostheses.
2. Description of the Related Art
Orthopedic prostheses are commonly utilized to repair and/or replace damaged bone and tissue in the human body. For example, a knee prosthesis may include a tibial component and/or a femoral component that replace damaged and/or destroyed bone in the tibia and/or femur and promote articulation similar to the natural, anatomical articulation of the knee joint.
In a natural knee, internal rotation of the tibia occurs when the knee is flexed from full extension (i.e., zero degrees flexion) to about 20 degrees flexion, and, conversely, external rotation of the tibia occurs when the knee is extended from about 20 degrees to full extension. This internal/external rotation is known as the “screw home” mechanism. The screw home mechanism is driven in part by the difference in the radii of curvature of the medial and lateral condyles of the distal femur. The externally rotated orientation of the anatomic tibia in full extension results in tightening of cruciate ligaments and “locks” the knee against internal/external rotation at the tibia-femur interface. The screw home mechanism thereby promotes stability of the tibia with respect to the femur when the knee is extended or slightly flexed.
In the final stages of knee extension, the tibia rolls anteriorly and the posterior cruciate ligament (PCL) elongates, causing translation of the femur relative to the tibia at the tibia-femur interface. The lateral side of the distal femoral articular surface experiences a relatively larger anterior translation as compared with the medial side of same. This anterior movement of the lateral distal femoral articular surface occurs during the last 20 degrees of knee extension results in external rotation of the tibia, and forms the basis for the screw home mechanism. Once in the extended position, internal/external rotation of the tibia is substantially prevented.
When the natural knee begins to flex from a position of full extension, the lateral side of the distal femoral articular surface translates posteriorly, elongating the anterior cruciate ligament (ACL). The lateral femoral articular surface experiences a relatively larger posterior translation as compared with the medial side of same. This posterior movement of the lateral distal femoral articular surface occurs during the first 20 degrees of knee flexion, and reverses the screw home mechanism. Once the knee is sufficiently flexed, such as about 20 degrees, internal/external rotation of the tibia is once again permitted.